The Dancing Mouse: A Study in Animal Behavior

by Robert Mearns Yerkes

POSITION OF WHITE CARDBOARDS FOR A SET OF 150 TESTS

SERIES 1 2 3 4 5 6 7 8 9 10

Preference
A l r l r l r l r l r
B r l r l r l r l r l

1 r l r l r l r l r l
2 l l r r l r l l r r
3 r r l r l l r l r l
4 l l l r r r l r r l
5 r l r l r l r l r l
6 l l r l r r l r l r
7 r l l l r r r l r l
8 r r l l r l r l r l
9 r r r l l l r l r l
10 l l l l r r r r l r
11 r l r r r l l l r l
12 r l r l r r l l r l
13 r l r l l l r r r l
14 l l l l r r r r l r
15 r l r r r l l l r l

It is to be noted that in the case of each series of ten tests the white cardboard was on the left five times and on the right five times. Thus the establishment of a tendency in favor of one side was avoided. The irregularity of the changes in position rendered it impossible for the mouse to depend upon position in its choice. It is an interesting fact that the dancer quickly learns to choose correctly by position if the cardboards are alternately on the left box and on the right.

The prevalent, although ill-founded, impression that mice have an exceedingly keen sense of smell might lead a critic of these experiments to claim that discrimination in all probability was olfactory rather than visual. As precautions against this possibility the cardboards were renewed frequently, so that no odor from the body of the mouse itself should serve as a guiding condition, different kinds of cardboard were used from time to time, and, as a final test, the cardboards were coated with shellac so that whatever characteristic odor they may have had for the dancer was disguised if not totally destroyed. Despite all these precautions the discrimination of the boxes continued. A still more conclusive proof that we have to do with brightness discrimination, and that alone, in these experiments is furnished by the results of white- black tests made with an apparatus which was so arranged that light was transmitted into the two electric-boxes through a ground glass plate in the end of each box. No cardboards were used. The illumination of each box was controlled by changes in the position of the sources of light. Under these conditions, so far as could be ascertained by critical examination of the results, in addition to careful observation of the behavior of the animals as they made their choices, there was no other guiding factor than brightness difference. Nevertheless the mice discriminated the white from the black perfectly. This renders unnecessary any discussion of the possibility of discrimination by the texture or form of the cardboards.

Since a variety of precautionary tests failed to reveal the presence, in these experiments, of any condition other than brightness difference by which the mice were enabled to choose correctly, and since evidence of ability to discriminate brightness differences was obtained by the use of both reflected light (cardboards) and transmitted light (lamps behind ground glass), it is necessary to conclude that the dancer possesses brightness vision.

CHAPTER VIII

THE SENSE OF SIGHT: BRIGHTNESS VISION (Continued)

Since the ability of the dancer to perceive brightness has been demonstrated by the experiments of the previous chapter, the next step in this investigation of the nature of vision is a study of the delicacy of brightness discrimination, and of the relation of the just perceivable difference to brightness value. Expressed in another way, the problems of this portion of the investigation are to determine how slight a difference in brightness enables the dancer to discriminate one light from another, and what is the relation between the absolute brightnesses of two lights and that amount of difference which is just sufficient to render the lights distinguishable. It has been discovered in the case of the human being that a stimulus must be increased by a certain definite fraction of its own value if it is to seem different. For brightness, within certain intensity limits, this increase must be about one one-hundredth; a brightness of 100 units, for example, is just perceivably different from one of 101 units. The formulation of this relation between the amount of a stimulus and the amount of change which is necessary that a difference be noted is known as Weber's law. Does this law, in any form, hold for the brightness vision of the dancing mouse?